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(a) Genetic variation and human reproduction

Reminders: A chromosome as a thread-like structure of DNA, carrying genetic information in the form of genes.

A gene is a length of DNA that codes for a protein. An allele as a version of a gene.

Reminder that in the biological science of genetics, inheritance is the transmission of genetic information from one generation to the next generation by chromosomes of DNA.

Summary of some definition for genetics

All body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs to fulfil its specific function.

A haploid nucleus is a nucleus containing a single set of unpaired chromosomes, e.g. in gametes (sex cells).

A diploid nucleus is a nucleus containing two sets of chromosomes, e.g. in human body cells, which contain a pair of each type of chromosome, so the human diploid cell has 23 pairs of chromosomes.

Genetics is the study of heredity and the variation of inherited characteristics.

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Know and understand that sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.

Know and understand that in human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex.

All human cells have 22 matched pairs of chromosomes but the 23rd chromosome is different between the sexes.

Diagrams of chromosomes from micrographs

(i) In the above diagram the pairs of chromosomes are shown joined together by a centromere during duplication to give the X shape. (image adapted from shutterstock.com 701025034) e.g. see the cell division by meiosis diagram below.

(ii) In this diagram the pairs of chromosomes are shown as separate chromatids. (image adapted from the US National Library of Medicine) This profile of a set of chromosomes is an example of a karyotype.

22 pairs of the chromosomes look the same in both males and females and most are roughly X shaped when one is copied.

They are numbered 1 to 22 in decreasing size. However, for the 23rd pair of sex chromosomes, men have an X and Y chromosome (XY on the diagram) and women have two X chromosomes (XX on the diagram)

The lack of the Y chromosome, i.e. the XX gene combination causes female characteristics to develop in the embryo, eventually producing an adult female.

The Y chromosome carries a gene that causes male characteristics to develop in the embryo, eventually producing an adult male.

Male cells in the testes and female cells in the ovary divide by meiosis - illustrated below,

Diagrammatic reminders of sexual reproduction including meiosis and fertilisation.  For more details on meiosis see Cell division - cell cycle - mitosis, meiosis, sexual/asexual reproduction, binary fission  gcse biology revision

In sexual reproduction, the parents (mother and father) produce gametes (egg and sperm reproductive cells).

Each gamete only has one copy of each chromosome, unlike pairs of chromosomes in all other cells.

Therefore the gametes have only one version of each gene, an allele.

In producing offspring from fertilisation, the chromosomes from a male gamete (sperm) mix with the chromosomes from the female gamete (egg) to produce the full compliment of pairs of chromosomes - two alleles for each gene.

When sperm is made the X and Y chromosomes are drawn apart in the first meiotic division.

Therefore, in the first stage of the meiosis of sperm cells, there is a 50% chance of having an X or Y chromosome in the new sperm cell.

All egg cells will always have one X chromosome.

Therefore on egg fertilisation there is a 50% chance of an XX or XY combination ie a 50% chance of being male or female (see table and diagram below).

Note use of the word 'chance'. These 'chances' are the probable outcome of many sexual reproductions.

In any data set, because of the random combinations of the gametes (from available possibilities), the outcome is unlikely to be perfectly 1:1, but more likely 48% : 50% (0.48 : 0.52) or 51% to 49% (0.51 : 0.49)

So bear this idea in mind when ratios like 1 : 3 etc. are quoted i.e. in reality as well as the possibility of 1.00 : 3.00, for other data sets it might be 0.97 : 3.03 or 1.02 to 2.98).

Tabular and diagrammatic methods of obtaining these probability ratios are described in section (b).

Footnote

My good Irish wife Molly, had a cousin who has seven sons and no daughters!

So much for statistical probability and the apparent dominance of the XY genotype here!

 

Note: In a fertilised egg, multiple cell divisions occur by mitosis to produce all the huge number of cells a complex living organism like ourselves needs to grow and develop.

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(b) Revision - Methods of constructing two types of genetic diagrams

1. Punnett square genetic diagram for determination of gender

To find the probability of phenotype outcomes you can construct a Punnett square deduced from 'crossing' the different genes or chromosomes.

In this case you construct a genetic diagram or 'chart' to show the possible outcomes from XX crossed with XY.

You put the possible gametes from the female above the ('yellow') square (X and X) and the possible male gametes (X and Y) down the left side of the square.

You then fill in the matching genotype pairings giving XX, XX, XY and XY.

As you can see, on average there are two male phenotype and two female phenotype outcomes.

In other words, a 2 in 4 (50%) chance of a baby being a boy or a girl.

These outcomes can also be shown as another type of genetic diagram shown below.

2. Circles with connecting lines genetic diagram for determination of gender

You can also construct a 2nd type of genetic diagram using circles and connecting lines.

At the top are the parents indicating the phenotype and genotype.

Below that you show the possible gametes that can be formed, X or Y.

One gamete from parent a combines with one gamete from parent b in fertilisation.

You then use connecting lines to show how the chromosomes can combine, XX or XY.

Finally, the bottom row of circles show the genotypes of the offspring, to which you can add the phenotype, XX = female and XY = male.

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(c) Genetic fingerprinting

Reminders

Know and understand that some characteristics are controlled by a single gene.

Each gene may have different forms called alleles.

Know and understand that an allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.

This is important when interpreting genetic diagrams (see above with the genetic disorder polydactyly).

Know and understand that an allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.

This is important when interpreting genetic diagrams (see with the genetic disorder cystic fibrosis).

Know and understand that a gene is a small section of DNA.

Genes code for specific proteins and the type of cell they form part of.

Know and understand that each gene codes for a particular combination of amino acids which make a specific protein.

Know and understand that each person (apart from identical twins) has unique DNA - a genetic fingerprint.

DNA fingerprinting is a technique that simultaneously detects lots of sections in the human genome to produce a pattern unique to an individual.

This is a DNA fingerprint and the probability of having two people with the same DNA fingerprint that are not identical twins is very small indeed.

(Actually, because of the chance of imperfect DNA replication, even identical twins don't have a perfect match of their whole genome - but the phenotype outcomes are so close, the term 'identical twins' is still appropriate, since it is difficult to detect their differences.)

Know that this can be used to identify individuals in a process known as DNA fingerprinting.

Use DNA genetic fingerprinting 1. Forensic science

The technique is used in forensic science and your DNA can be checked against a database of previous suspects or convicted criminals!

DNA from samples of human origin from a crime scene can be compared with the DNA of a suspect believed to have committed a crime, and of course eliminate innocent people!

Use of DNA genetic fingerprinting 2. Archaeology

It is also used in archaeology to try and establish the original of ancient bodies and bones!

All you need is a sample of blood, hair, semen or skin from a body or crime scene.

It can also be used to identify if an individual is a relative of another.

As I was working on this page in 2013, the bones of King Richard III had been found by archaeologists in the City of Leicester, England. Chromosomal DNA was extracted from the bones and compared with one of the few known descendents of his family (a man in Canada, I think?) and a family match established. The bones showed that Richard III had a deformed back ('hunchback'), but you didn't need DNA to confirm that!

Since writing the above paragraph. on re-visiting Leicester, I took a photograph of the DNA evidence for confirming the bones found were those of Richard III (image below from the exhibition in the medieval Guildhall in Leicester from the work done by Leicester University).

They compared the mitochondrial DNA of Michael Ibsen and a 2nd matrilineal (lineage 2), with that of DNA extracted from the bones of Richard III. See the diagram below.

You can see the matching base peaks (colour coded) for the specific and characteristic sequence based on the four bases G (guanine), A (adenine), C (cytosine) and T (thymine) found in the structure of the compared DNA molecules of the individual genomes.

The sequence reads in sections such as ...GAACAAGCTATGTA.... etc.

Use of DNA genetic fingerprinting 3. Genealogy

Genetic fingerprinting can be used to identify if one person is related to another.

e.g. determination of the parent or parents of a child.

It has been used to identify children separated at birth, and re-united by their DNA profiles.

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Learning objectives

Know how and why sexual reproduction gives rise to variation and understand the gamete (sex) cells divide by meiosis.

Know how sex is determined in human reproduction from the presence of XX or XY chromosomes.

Be able to construct Punnett square tables for determination of gender.

Be able to a genetic diagram to show how gender is genetically determined.

Know and understand what genetic fingerprinting is.

Be able to describe how DNA genetic fingerprinting is used in forensic science, archaeology and genealogy

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